Large circumference loop antennas



mich 1, i9? P. s. CARTER A 297855395 LARGE cmcUMFERENcE Loop ANTENNAS Original Filed Jan. 9, 1946 ATTORNEY LARGE CIRCUMFERENCE L? ANTENNAS Philip S. Carter, Port .leii'ersom N. Y., assigner, by mesne assignments, to the United States of America as represented by the Secretary of the Army @riginal application January 9, 1946, Serial No. 639,998, now Patent No. 2,615,134, dated October 21, 1952. Divided and this application February 23, 1952, Serial No. 273,041

12 Claims. (Cl. 343-741) The invention relates to ultra short wave antennas and, more particularly, to loop antennas having dimensions large in terms of the operating wavelength.

This application is a division of U. S. patent application Serial No. 639,998 filed January 9, 1946, and since issued as U. S. Patent 2,615,134 on October 21, 1952.

An object of the invention is `the improvement of the band width of antennas.

Another object of the invention is improving the etliciency of short wave antennas.

Still another object of the invention is the provision of an antenna which may be mounted on an airplane without presenting objectionably large Wind resistance.

A further object of the invention is the provision of an airplane antenna which is less hazardous to ground crew personnel than previously known types.

Still a further object of the invention is Vthe provision of an antenna which, if desired, produces a non-directional radiation pattern for broadcasting use.

Still a further object of the invention is the provision of an antenna which is mechanically stable and electrically grounded.

Still another object of the invention is the provision of a loop antenna which may be energized from a transmitter by a coaxial line without the use of line balance converters or similar matching structures.

The foregoing objects and others which may appear from the following detailed description are attained by providing a loop antenna, having a circumference of the order of one or a few wavelengths at the operating frequency. The loop is preferably a single turn arrangement using a large diameter conductor. The antenna may be coupled to a coaxial transmission line and the antenna itself may utilize elements of the said coaxial transmission line as element-s of the loop construction. ln a modified form a half loop working against a metallic ground plane may be employed.

'l` he invention will be more fully understood by reference to the following detailed description and the accompanying drawing in which:

Fig. l illustrates an embodiment of the invention wherein a plurality of .horizontal loops are vertically stacked to produce a more uniform radiation pattern in the horizontal plane.

Fig. 2 illustrates in simplified diagrammatic form a modification of the invention utilizing a half loop operating against a metallic ground plane.

Fig. 3 illustrates a modification of the arrangement shown in Fig. 2 whereby the band width of the antenna is increased.

Figs. 4, 5 and 6 illustrate further modifications of the method of feeding the antenna shown in Fig. 2.

Fig. 7 illustrates the horizontal and vertical directivity patterns of a horizontal loop antenna, utilizing the principles ofthe invention.

Fig. 8 is a further field strength pattern of the invention; and

States Patent O ICC Fig. 9 is a family of curves illustrating the relationship between the radiation resistance of a loop antenna versus the loop circumference in wavelengths.

Referring now to Fig. l, there is shown a construction of the loop antenna of the invention whereby it may be utilized for broadcast transmission or reception. Two large diameter conductors 36 and 32 are bent into nearly closed circles, each having a circumference of two wavelengths. Between adjacent ends of loop conductor 32 is connected the vertical central two wire balanced transmission line TLB by means of conductors 33 and 3d parallel to a radius of loop 32 and preferably in the plane of the loop. The balanced transmission line TLB continues vertically a distance equal to an odd multiple of a quarter wavelength to the center of :loop 30. lt is there connected to the opposing ends of loop conductor 3f) by means of conductors 35, 36 parallel to a radius of the loop of conductor 3d. Transmission line TLB is twisted between loops 3l) and 32 so that the two radii mentioned above lie in vertical planes intersecting along the vertical axis of the antenna at an angle of forty-live degrees. Due to the forty-five degree displacement and the phase quadrature feeding relationship obtained by the quarter wave spacing between loops 3@ .and 32, perfect circle distribution of the radiated energy is attained in the horizontal plane. This may be demonstrated as follows:

From one loop alone, the field E is given by the relationship E1=A cos 2gb, where (p is a measure of the angular displacement around the center of the loop in the horizontal plane. The second loop alone gives a field E2 determined by the expression:

E2=iJA cos 2(bi45)=1J/l sin 2p (l) Adding the two fields together, the total field is:

Et=A (cos 2pm] sin 2gb)=/ie;i2 (2) Therefore, the field f |Et| :A (3

(lEtl indicates the magnitude of Et.)

Though only one method of energizing; the antenna of Fig. l is shown, any scheme giving a balanced feed may be used. Though the loops in Fig. l are shown as having a circumference of 2A, a pair of loops having a circumference of lk may be used, in which case, they are fed in phase quadrature, by thesame method as for the 2k loop, but the feed points are now spaced around the circumference. A similar mathematical consideration shows that Ithe field is again perfectly circular. Any number of antennas may be vertically stacked in order to obtain as much vertical directivity as desired with uniform horizontal field strength.

In Fig. 2 there is shown a modified form of the invention which is particularly useful on airplanes. The antenna comprises a conductor 60 bent in the form of a half loop having a semi-circumference of one half of the operating wavelength and connected at point el to a conductive ground sheet 5S. At the opposite end of the 'semi-circumference the inner conductor .il of transmission `line TL is connected to the semi-loop conductor ed. The outer sheath l0 of transmission line TL is connected to the ground sheet 58. Ground sheet S8 may be the outer surface of the fuselage of an airplane or it may be an upper or lower wing surface as desired.

As indicated by arrow F the antenna of Fig. 2 has a maximum of radiation in the direction along a line from the feed point toward point 61. However, the radiation minimum in the right angle direction is not greatly less than the maximum. Such an arrangement provides an airplane antenna having a low wind resistance. Also, due to its small extension from the surface of sheet 58, and shape, ground crew personnel are much less liable 3 to injuries than when the common whip antenna is used. Furthermore, the smooth outline of the antenna and its small projection render it less likely to become entangled with cables which may be employed around the landing field.

The modification show-n in Fig. 3 adds a sleeve feed `to the antenna of Fig. 2 to widen the band width. Her the transmission line TL .extends above .ground '9' a distance of the order of one Aeighth of the op wavelength in the form of a sleeve 'tl and an .i l doctor 7l. The inner conductor 71 is directly conne in series between semi-loop di? and inner conductor of transmission line TL. The inner conductor t p etcrably has a diameter less vthan the diameter of conductor .ll Thus, the 4eighth wave secion ce by 7d, 7i is arranged to have a characteri of a value lying `hetween the impedance of mission line andV the impedance of semi-loop antenna itself. Otherwise, the construction of the antenna of Fig. 3 is the same as that .of Fig. 2..

Similarly, the general construction of the furth "i modification Shown in Fig. 4 resembles that shown in Fig. 2, the only vdistinction being in the manner of feeding the antenna. Here the inner conductor il of transmission line TL is connected to the free end of semicircular conductor dit. The end of conductor ed is hollow a distance equal to approximately a quarter of the operating wavelength from the free end. Supplemental conductor 62 is concentrically .arranged within the hollow portion and is connected to ground plane 58 near point of emergence of inner conductor 11 of transmission line TL and connected .to semi-circular conductor 69 at the bottom of the hollow portion.

Thus a quarter wave section of transmission line -is placed effectively in shunt with the antenna at the feed point. This acts as a compensation circuit to widen the frequency band. At the ymidband frequency where the length is exactly a quarter wavelength the impedance is infinite but when the frequency is .higher than that of the midband frequency it presents a capacitive reactance in shunt with the feed point, thus compensating for the inductive reactance of the antenna. At a frequency somewhat lower than the midband frequency .the shunt line presents an inductive reactance in shunt with the antenna compensating for the capacitive reactance of the antenna.

Fig. 5 shows a further modification of the invention. Herein the semi-circular conductor 94) .has an overall length of one quarter of the operating wavelength. Thus, the semi-loop taken in conjunction with the electrical image formed by ground plane t; .acts as a loop having a circumference of a half wavelength rather than a one wavelength circumference loop as previously described. Conductor 9i? is hollow through nearly its entire length. Coaxially arranged within hollow conductor 90 is an extension 63 of the inner conductor 11 of Vtransmission line TL. The interior quarter Vwave section acts as a compensation circuit to widen the frequency band. By suitably varying the length of extension conductor 63 within conductor 9i) from the point of entrance to shorting plate 64, the reactive component of the antenna may be tuned out over at least a portion of the operating band. The effect of the change in circumference upon the characteristics of the antenna will .be discussed later by reference to the impedance characteristics and directivity patterns.

The half loop antenna shown in Fig. .6 is again a half wave semi-loop. The inner conductor 11 of transmission line TL extends half way around the semi-circumference of the loop where it meets and is connected to conductor 69. it is surrounded by a curved supplemental sleeve 92 insulated from both `conductor `6l) and ground sheet 53. Thus, the effective feed 'point of the loop is moved around to point 100 instead of Jbeing closely ad- 75 jacent to the ground plane Vas in previously discussed modifications.

A mathematical development of the theory and properties of large loop antenna is given in detail in the parent U. S. patent application 539,998, filed .lanuary 9, 1946, since issued as U. S. Patent 2,615,134, and will not be repeated herein. In this application, as in the parent application, a vertically polarized wave is defined as one whose magnetic vector is horizontal, that is, the electric vector lies ina vertical plane containing the direction of propagation or ray. The object of this definition is to avoid limiting the direction of propagation to the horizontal. Also in this application the current distribution in the loop is assumed to be sinusoidal as shown in Fig. 12 of the parent application. This is a close approximation to the actual distribution except at the current minimum points where the actual current is finite rather than zero. Regardless of the circumference of the loop the current `distribution will always he symmetrical with respect to the point directly opposite the feed point.

Fig. 7 curve c, shows the horizontal field strength pattern, that is, the pattern in the plane of the antenna for a loop having a half wave circumference. It will be noted that, when the loop is used for transmitting, the radiation is greater in the direction of a diameter through the feed point. Fig. 7, curve d, shows the vertical pattern in the plane =zero, that is, with the vertical plane passing through the feed point. There is no vertically polarized field (Eg) in this plane. The vertical patterns for both horizontally polarized (Es) and vertically polarized (E0) radiation in the vertical plane =90 are shown in Fig. S. In this plane, that is, the plane at right angles to that through the feed point, the horizontally polarized radiation is a maximum in the horizontal direction and zero vertically while the vertical polarized radiation E9 is zero horizontally and a maximum vertically. In the regions betewen horizontal and vertical, the radiation is elliptically polarized since E@ and Es are in phase quadrature.

Where the circumference of the loop is increased to one wavelength, the horizontal pattern is a simple figureeight or cosine curve with maxima along the diameter passing through the feed point. The `vertical pattern in the plane =0 (vertical plane passing through the feed point) is the same as that shown in Fig. 7c. The radiation is a maximum in the `vertical direction while the E9 (vertically polarized radiation) is Zero everywhere in this plane. The verticalradiation pattern in the plane =90, that is, the lvertical Vplane perpendicular to the plane passing `through the feed point, is a simple ligure-eight or cosine curve. All radiation is vertically polarized (E8 radiation), the horizontally polarized radiation (Ei) being zero everywhere. It will be noted that this situation is quite different from the half wave circumference loop which radiates considerable horizontally polarized energy in this plane.

Where a loop has a circumference equal to a multiple of a wavelength, the horizontal patterns are simple cos mqb curves where mis the integral multiple of the wavelength in the circumference. Thus, the figure-eight pattern for a loop of one wave circumference becomes in the case of two wave circumference a four-leaf clover pattern.

The varia-tion in radiation resistance with loop circumference is shown in detail in Fig. 9 for loops up to two wavelengths in circumference with the value for three wavelengths also indicated. Curve a of Fig. 9 indicates the total variation in radiation resistance with a variation of the loop circumference while curves b and c indicate respectively the horizontally and vertically polarized radiation resistance components .of curve a. The values of. the radiation resistance between two wavelengths and three wavelengths are not .shown .indetail `but the general shape of the curve is well indicated in the area between one and two wavelengths. It should be noted that for circumferences greater than a half wavelength, the radiation resistance is of -the order of that of a half-wave dipole, reaching a maximum of over 160 ohms in the vicinity of one and one-quarterV wavelengths vcircumference. The radiation efficiency, it will therefore be noted, is very high in contrast to the extremely low efficiency of small loops.

The invention claimed is:

l. A short wave antenna system including a conductor curved to follow half the circumference of a circle having a circumference of a multiple including unity of a wavelength at the operating frequency, a conductive ground sheet lying transverse to the plane of and along a diameter of said circle, one end of said conductor being connected to said ground sheet, there being a gap between the other end and said sheet.

2. A short Wave antenna including a conductor curved to follow half the circumference of a circle having a circumference of a multiple including unity of a wavelength at the operating frequency, a conductive ground sheet lying transverse to the plane of and along a diameter of said circle and being connected to one end of said curved conductor, and a coaxial transmission line having an outer shell and an inner conductor, said outer shell being connected to said ground sheet and said inner conductor being coupled to the other end of said curved conductor, said outer shell extending from said ground sheet a distance equal to one-eighth of the operating wavelength.

3. A short wave antenna system including a conductor curved to follow half the circumference of a circle having a circumference of a multiple including unity of a Wavelength at the operating frequency, a conductive ground sheet lying transverse to the plane of, and along a diameter of said circle and being connected to one end of said curved conductor, and a coaxial transmission line having an outer shell and an inner conductor, said outer shell being connected to said ground sheet and extending from said ground sheet a distance equal to one-eighth of the operating wavelength, said inner conductor extending to the other end of said curved conductor, the inner conductor of said transmission line within said extending part having a smaller diameter than Ithe remainder whereby said extending portion acts as an impedance matching section.

4. A short wave antenna system including a conductor curved to follow half the circumference of a circle have ing a total circumference equal to a multiple including unity of a wavelength at the operating frequency, a conductive ground sheet lying transverse to the plane of and along a diameter of said circle, one end of said conductor being connected to said ground sheet, there being a gap between the other end and said sheet, and a coaxial transmission line having an outer shell and an inner conductor, said outer shell being connected to said ground sheet, said inner conductor being connected to the end of said curved conductor, said curved conductor being hollow from its free end for a distance equal to one quarter of the operating wavelength, and an auxiliary conductor connected to said ground sheet and extending within the hollow portion of said curved conductor.

5. A short wave antenna system including a radiation transferring member in the form of an elongated conductor arranged to lie along the perimeter lof a half of circle having a circumference of at least one operating wavelength, 'at least a portion of said conductor being hollow, la further conductor disposed in said hollow portion and connected at one end to the rst said conductor, a conductive surface member connected to one end of and arranged with respect to ithe rst said conductor to form a gap therebetween, and a coaxial transmission line having a sheath conductor connected to said conductive surface member and an inner conductor coupled across said gap to the other end of said elongated conductor, said further conductor being connected at its other end to said conductive surface member, thereby to provide a substantially omnidirectional directivity pattern.

6. A short wave antenna system including a radiation transferring member constituted by an arcuately formed conductor arranged in the form of a semi-circle, the circumference of a complete circle having the same radius being at least one operating wavelength, at least a portion of said conductor being hollow, a further conductor disposed in said hollow portion and connected at one end to the first said conductor, said further conductor being connected at its other end to said conductive surface member, a conductive surface member connected to one end 'of and arranged with respect to the arcuately formed conductor to form a gap therebetween, and a transmission line having two conductors, one of said tnansrnission line conductors being connected to said conductive surface member and the other conductor being coupled across said gap to the other end of said arcuately formed conductor.

7. A short wave 4antenna system including a radiation transferring member constituted by an arcuately formed conductor arranged in the form of a semi-circle to lie along half of the circumference of a circle having a circumference of at least one operating wavelength, at least a portion of said conductor being hollow, a conductive surface member connected to one end of and arranged with respect to the arcuately formed conductor to form a gap between said member and the other end of said conductor, a further conductor disposed in said hollow portion and connected at one end to the arcuately formed conductors and the other end connected to said conductive surface member, and a transmission line having a sheath conductor connected to said conductive surface member and an inner conductor coupled across said gap to said other end of said arcuately formed conductor, thereby to provide a substantially omnidirectional directivity pattern.

8. A short wave antenna system including a radiation transferring member constituted by arcuately formed quadricircular conductors arranged in the form of a semicircle of circumference of at least one operating wavelength and with a gap between said conductors, at least one of said conductors being hollow, a conductive sur face member connected to one end of the other of said conductors and arranged with respect to said conductors to form a second gap between said member and the end of said hollow conductor, and a coaxial transmission line having inner and sheath conductors, means to connect said sheath conductor to said conductive surface member, and means to extend said inner conductor across said second gap through said hollow quadri-circular conductor and to connect said inner conductor to the other end of said other arcuately formed conductor opposite to the end of the hollow conductor, the inner conductor of said transmission line passing through said hollow conductor for a distance at which the impedance of said quadricircular conductors is effectively matched to the impedance of said transmission line, thereby to provide a substantially omnidirectional directivity pattern.

9. A short wave ant-enna for use in conjunction with a conductive -surface element, including at least one elongated conductive element at least a quarter wavelength long at the operating frequency and bent to lie on the semi-circumference of a circle having a circumference of at least a half wavelength at said frequency, means to connect said elongated conductive element to said conductive surface element and to maintain the same in a plane normal thereto, a two conductor transmission line having one conductor thereof connected to the other end of said one elongated conductive element and the other conductor of said transmission line connected to said conductive surface element.

l0. A short wave antenna for use in conjunction with a conductive surface element including at least one elongated conductive member at least a half wavelength long ayrsasse at the operating frequency and bent to 'lie on the semicircurnference 0f a circle having a circumference of at least a wavelength at said frequency, means to connect one end of said .one conductive member` to said conductive surface element, a two conductor transmission 1in-e and means to connect one conductor theretof to the other end of said one conductive member, and to connect the other conductor to said conductive surface element.

11. A short wave antenna system including a conductor curved to follow half the circumference of a circle having a circumference of a multiple of quarter wavelengths at the operating lfrequency, a conductive ground sheet lying transverse tothe plane of and along a diameter of said circle, one end of said conductor being connected to said ground sheet, there being a gap between the otherend and said sheet, at least a portion of said conductor being hollow, and a coaxial transmission line having an outer shell connected to said ground sheet and an inner conductor passing across said gap and within the hollow portion of said conductor and being connected to said curved conductor therein.

12. A short wave antenna system including a radiation transferring member constituted by an arcuately formed conductor arranged in the form of a semi-circle,

the circumference of a complete circle 'having the same radius being at least one operating Wavelength, ra condnctive 4surface member connected to one end of and arranged with Arespect to the yother end of 4the arcuately formed conductor to form a gap therebetween, and a transmission line having two conductors, one of said transmission line conductors being connected to said conductive surface member and the other conductor being coup-led across said gap yto said other end of said arcuateiy formed conductor.

References Cited in the tile of this patent lUNITED STATES PATENTS 

